Shulou(Shulou.com)11/24 Report--

Original title: "Qatar is so hot, fancy cooling method to understand?"

This World Cup came a little later than usual,

Without him,

Qatar is so hot,

Even if it's winter instead of summer,

The local temperature is still amazing. Isn't it summer?

Doha Temperature| Photo source: https://weathernew.pae.baidu.com Looking back at Beijing, this cold wave, this cooling,

This is the winter in my memory!

From pitch structure to air conditioning installation,

To provide a comfortable environment for the players,

Qatar has also made great efforts to cool down.

3D Printed Bayside Stadium| Photo source: qf.org When it comes to cooling down, Xiaobian won't be sleepy.

Today, Xiaobian will take you to have a look.

To get low temperatures,

what people have come up with.

01, ice cubes, or ice cubes In that era when there was not so much technology and hard work, people wanted to cool down and could only rely on nature's gift-ice cubes.

Using ice to cool down, the main problem is how to get ice in the hot summer days. In this regard, the ancient people had two ways to solve: the ice method and saltpeter ice method.

First of all, the ice hiding method, as the name implies, is to store ice. It is recorded in the Book of Songs, Guofeng, Bingfeng and July: "On the second day, the ice is chiseled, and on the third day, it is received in Lingyin." "The description is the scene of ancient Chinese people using ice cellars to store ice in winter.

Map of Ancient Ice Mining and Storage| Not only China, but other countries also have a history of ice storage, such as around 400 BC, Persia has ice cellar facilities such as yakhchal.

Yakhchal |The saltpeter ice making method originated in the Tang Dynasty. When saltpeter is thrown into water, the water will slowly cool down and freeze into ice. The principle is very simple, that is, the use of potassium nitrate dissolution heat absorption.

The discovery of saltpeter ice contributed to the rise of cold drink industry in ancient China, and there were special cold drink shops in Song Dynasty. People also gave full play to their creativity and invented a series of drinks such as "crystal sugar ice seed," iced sour plum soup,"snow bubble bean water,""snow bubble plum wine," etc., such as iced sour plum soup is very popular even now.

02, towards low temperature for a long time, ice in the field of human refrigeration alone, at this time the low temperature is only 0℃.

Until the discovery of the modern liquid separation air method, people had more choices in refrigeration methods, and the temperature finally reached the field of minus Celsius. Since then, the development of refrigeration has two directions: one is the optimization of refrigeration methods between zero and a few degrees below zero, such as for air conditioners and refrigerators; the other is the continuous pursuit of lower temperatures and the study of the properties of substances at low temperatures. This article focuses on the second direction.

Linde air liquefier| Liquid gases are used in refrigeration much like ice cubes, which are cooled by phase change refrigeration. Phase change refrigeration is the use of refrigerant phase change heat absorption to refrigeration, such as ice melting to absorb heat, this is a phase change heat absorption process.

Now the most common refrigeration method in the laboratory is to use liquid nitrogen and liquid helium to do phase change refrigeration. Liquid hydrogen and other liquid gases have been used in laboratories, but they are not as safe as liquid nitrogen and liquid helium. In addition, the maturity of the extraction process of the two gases has gradually eliminated other gases.

At atmospheric pressure, the boiling point of liquid nitrogen is 77K (-196℃), and the boiling point of liquid helium is 4.2 K (-268.8℃), which means that after the existence of liquid helium and liquid nitrogen, even without pressure regulation and other treatment methods, the temperature can be reduced as low as 4.2 K, which is enough for many novel physical properties to be studied.

After receiving liquid nitrogen and liquid helium, many critical temperature phenomena have criteria for discrimination. The superconducting transition temperature of this material is above 77K. Okay, so it's a high-temperature superconductor, because it only needs liquid nitrogen to make it superconducting, and it doesn't need other complicated and expensive refrigeration systems, which is really friendly to researchers.

In addition to phase change refrigeration using liquid gases, there are also phase change refrigeration using solid refrigerants, expansion refrigeration using throttling and degassing processes, thermoelectric refrigeration, radiation refrigeration and adsorption refrigeration. These methods have their own advantages and disadvantages, have different application scenarios, and have made contributions to human cryogenic research.

Goal: Absolute Zero The third law of thermodynamics tells us that absolute zero is impossible. Can we get closer to that temperature?

We talked about using liquid gas phase change refrigeration, and when the temperature drops to 0.6 K, all other materials exist in solid form and cannot get lower temperatures. Helium is a special material that cannot be converted into a solid under normal pressure, so liquid helium can be used to achieve lower temperatures, but the lower limit of actual operation can only be 0.4K, and there is still a distance from 0.

But it doesn't matter, we have methods: magnetic refrigeration, dilution refrigeration, and laser refrigeration.

Magnetic refrigeration utilizes the magnetocaloric effect: magnetic ion systems emit heat and absorb heat in the process of applying and removing magnetic fields, respectively. When the system satisfies adiabatic demagnetization condition, Carnot cycle can be constructed to realize refrigeration.

Schematic diagram of magnetic refrigerator| At present, the refrigeration limit of magnetic refrigeration can reach mK level, but different from other methods, magnetic refrigerator can only make extremely cold, and magnetic refrigeration technology in high temperature region (above 20K) has not yet been mature. This is because adiabatic demagnetization and significant heat absorption can only be achieved at extremely low temperatures.

Dilution refrigeration technology was first proposed and perfected by London, and its principle is to dilute He by superfluid to absorb heat and realize refrigeration. Commercial dilution chillers can achieve extreme refrigeration of 5mK.

Dilution Refrigeration Flow Chart| If you think 5mK is still too high, don't worry, we also have laser cooling technology, directly to the temperature down to a millionth of Kelvin! However, this technology is currently only suitable for small-scale laboratory applications and has not yet been commercialized. The principle is very simple, that is, the use of scattering to slow down the atoms of matter, reduce the intensity of microscopic particle movement, so as to achieve cooling.

04, low temperature technology has been developing, never stopped Why continue to study low temperature, find ways to approach absolute zero?

Naturally, there are good things about low temperatures.

On the one hand, many states are revealed only below the transition temperature. For example, helium is the only gas that cannot be transformed into a solid at atmospheric pressure, and helium exhibits novel superfluid properties at temperatures below 2.18 K. For example, superconductivity, a material whose resistance suddenly disappears below a certain temperature, enters a superconducting state.

On the other hand, due to the thermal fluctuation of microscopic particles, many weak phenomena and particles are hidden and cannot be observed at high temperatures, which requires observation in low temperatures, such as fractional quantum Hall effect, one of the candidates for quantum computing.

The development of modern physics is inseparable from the maturity of cryogenic technology, and these novel physical properties will also promote people's exploration towards absolute zero.

References:

1. https://www.huxiu.com/article/719812.html

2. Low temperature development history and acquisition method-20210322000712.pptx-Original Power Document (book118.com)

3. Ice - Wikipedia

4. Cold drink_Baidu.com

This article comes from Weixin Official Accounts: Institute of Physics, Chinese Academy of Sciences (ID: cas-iop), author: Le Zi Superman

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